A heat-sensitive recording material generally has a substrate and a heat-sensitive recording layer formed thereon, and the heat-sensitive recording layer contains, as main components, an electron-donating colorless or slightly colored dye precursor and an electron-accepting color developer. When a heat-sensitive recording material is heated with a hot head, a hot pen or a laser beam, the dye precursor and the color developer immediately react with each other to give a recorded image. Advantageously, the above heat-sensitive recording material permits recording with a relatively simple-structured device, and such a device is easy to maintain and makes almost no noise, so that the heat-sensitive recording material is applied to broad fields including measurement meters, facsimile machines, printers, computer terminals, labeling machines, ticket vending machines, and the like.
Particularly in recent years, such heat-sensitive recording materials have come to be used as financial recording sheets for bills and receipts of gas, water, electricity, etc., slips of automatic teller machines and various receipts.
As the use and demands of such heat-sensitive recording materials have expanded into various fields in various ways as described above, there has come to be demanded a heat-sensitive recording material which has high thermal response for giving sufficient recorded image in power saving and fast printing. The thermal response is greatly interrelated with properties of an electron-accepting compound.
However, the heat-sensitive recording material gives an image when heated. When a high thermal response is attained, there are caused defects that a recorded image is degraded and a ground fogging takes place to a greater extent when the heat-sensitive recording material is exposed under high-temperature or high-humidity conditions. Due to the degradation of the recorded image and the ground fogging, a contrast between a ground and the recorded image comes to be lost. It is therefore desired to develop a heat-sensitive recording material which has a high thermal response and high saturation density but which does not much cause the ground fogging and is excellent in ground/recorded image retainability.
Further, there is another possibility that a recorded image comes in contact with various chemicals, and the heat-sensitive recording material has come to be required to have chemical resistance against office necessaries such as a water-base ink, an oil-base ink, a highlighter, a cinnabar seal ink, an adhesive and diazo developer solution or cosmetics such as hand cream, hair tonic and a lotion and durability against a plasticizer contained in a vinylidene chloride film or a synthetic leather.
Further, there has come to be demanded a heat-sensitive recording material which has excellent retainability of a ground portion against exposure to sunlight outdoors and through a window and exposure to fluorescence lamp light indoors (retainability against light) in addition to basic properties such as high thermal response, a high color density and a whiteness of a ground. That is, there is demanded a heat-sensitive recording material which shows little discoloration in a ground portion and causes little deterioration of an image even when it is exposed to light such as sunlight or a fluorescence lamp light. When a ground portion is discolored, or an image is deteriorated, to a great extent, there is caused a problem that reading is difficult due to a decrease in contrast between the ground portion and the image.
As a heat-sensitive recording material having a high thermal response and having excellent ground/recorded image retainability, JP-B-5-13071 discloses a heat-sensitive recording material using a benzenesulfonamide derivative as an electron-accepting compound. However, this heat-sensitive recording material cannot be said to have attained a practically sufficient thermal response and recorded image retainability.
Further, JP-A-10-862 and JP-A-10-29969 disclose heat-sensitive recording materials excellent in recorded image retainability. However, this heat-sensitive recording material is insufficient in thermal response, so that it is difficult to cope with practically fast recording.
As a method for improving a heat-sensitive recording material in retainability against light, JP-A-50-104650 discloses a method in which an ultraviolet absorbent is added. Publication of Japanese Patent No. 2727234 and JP-A-7-47764 disclose that a benzotriazole derivative is selected as an ultraviolet absorbent and added. However, when such ultraviolet absorbents are added, the retainability of an image against light cannot be said to be sufficient although the retainability of a ground against light is found improved.
Meanwhile, it is desired in view of demands based on resources problems that a substrate for a heat-sensitive recording material contains a recycled paper pulp. JP-A-58-25986 and JP-B-7-85945 disclose that a recycled paper pulp is incorporated into a substrate for a heat-sensitive recording material.
A heat-sensitive recording material having the above substrate containing a recycled paper pulp cannot be said to have attained practically sufficient retainability of ground/recorded image. Further, from demands based on environmental and resources problems, it has come to be desired to incorporate a non-wood pulp into a substrate. All the non-wood fibrous materials that can be used as a raw material for a non-wood pulp grow for a very short period of time and can be used as the above raw material.